#TROPHIC CASCADE WOLVES FULL#

Furthermore, a recent modeling study has predicted there will be an aspen snag deficit during the second half of the 21st century corresponding to the aspen recruitment gap that was created during the long wolf-free period of the 20th century.

#TROPHIC CASCADE WOLVES FULL#

This may be explained by elks being more vulnerable to wolf attacks at higher elevations in winter due to relatively deep snowpacks. In addition, researchers have documented major behavioral effects whereby elk in YNP, under the risk of predation by wolves, have altered their habitat use, movements, group sizes, vigilance, and other traits.Įven with the occurrence of increased recruitment within existing aspen stands, full recovery of aspen to historical conditions may not be possible during the next few decades because most stands (approximately 2/3rds) have already died out and were lost due to heavy elk herbivory during the seven-decade period of wolf absence. Sometimes large-scale shifts in behavior due to predation risk may locally appear to be density effects. For example, in recent years elk have reduced their use of the high elevation winter range in and around the Lamar Valley compared to low-elevation winter range both in and out of the park. While more widespread aspen recruitment would suggest an increasing inﬂuence from a density-mediated trophic cascade because elk numbers have trended signiﬁcantly lower since wolf reintroduction, it is difﬁcult to separate density effects from behavioral effects because predation risk can be temporally dynamic and exist at multiple spatial scales, from a few meters to very large landscapes. For example, species such as aspen ( Populus tremuloides) and willows ( Salix spp.) were generally unable to successfully recruit young stems into the overstory on Yellowstone’s northern winter ranges, except in fenced exclosures. Since wolf reintroduction, Yellowstone northern ecosystems have responded as predicted by classic ecological theory with alternating biomass levels across adjacent trophic levels (i.e., more wolves, fewer elk with altered behavior, more plant biomass). In a system with three trophic levels involving predators, prey, and plants, predators can indirectly affect plant communities by inﬂuencing prey behavior and density, thus releasing plants from herbivory. The collapse of a tri-trophic cascade allowed elk ( Cervus elaphus) to signiﬁcantly impact wildlife habitat, soils, and woody plants. Once infected, the snails lose their appetite and reduce their grazing, which, in turn, permits algae to flourish.In Yellowstone, wolves ( Canis lupus) were extirpated from the park by the mid 1920s, absent for a period of seven decades, and reintroduced in the winters of 1995/1996. Case in point: larval trematodes -parasitic flatworms-that infect snails. They also caused a nonconsumptive trait-mediated indirect effect by scaring the elk so the ruminants ate less, which also contributed to tree recovery, but because infectious agents can be less than fully and immediately lethal, they can simultaneously consume their hosts and change their behavior, initiating what we call a consumptive traitmediated indirect effect.'īuck found this new category applied to 45% of the infectious agent case studies she discovered in her review. 'The wolves initiated a consumptive density-mediated indirect effect on aspen by reducing the population of elk. 'Our analysis found a hybrid type of indirect effect: explains Buck. The findings appeared in the journal Trends in Ecology & Evolution.

She curated 47 examples and categorized them into three different types, including a new paradigm not applicable to predator-prey cascades. Ripple, Distinguished Professor of Ecology at Oregon State University-who described trophic cascades in Yellowstone soon after the wolves were reintroduced-Buck surveyed the literature for studies of cascades initiated by parasites and pathogens. After a chance meeting at UCSB with William J. Parasites and pathogens are her specialty. Other catalysts exist, including infectious agents, such as parasites and pathogens, and that is where former University of California, Santa Barbara, National Science Foundation postdoctoral research fellow Julia Buck-now assistant professor in the Department of Biology & Marine Biology at the University of North Carolina, Wilmington -comes in. Yellowstone is a classic-but not singular -example of a predator-initiated trophic cascade. The absence of the wolves, an apex predator, had triggered the process. At Yellowstone, that particular cascade goes from wolves to elk to aspen.

Young aspens had been devastated and all but disappeared, courtesy of elk, in what is known as a trophic cascade-an ecological process that begins at the peak of the food chain and ripples downward. When gray wolves were reintroduced to Yellowstone National Park, they sparked a resurgence of aspen trees.

Trophic cascade wolves